Is Union a Type of Data Structure?


Larry Thompson

In programming, a union is a type of data structure that allows different types of data to be stored in the same memory location. It provides a way to interpret the same memory area as different types of data, depending on how it is accessed.

Why Use a Union?

Unions are useful when you need to work with data that can be represented in multiple formats or when you want to save memory by sharing storage for different types of variables. They are commonly used in low-level programming languages like C and C++.

The Syntax

The syntax for declaring a union in C is similar to that of a structure. You use the union keyword followed by the union name and a list of member variables inside curly braces.

union MyUnion {
  int integerValue;
  float floatValue;
  char charValue;

In this example, we have declared a union named MyUnion with three member variables: an integer, a float, and a character.

Accessing Union Members

To access the members of a union, you can use either dot notation or arrow notation. The choice depends on whether you are working with the union directly or through a pointer.

Dot Notation:

union MyUnion myUnion;
myUnion.integerValue = 42;
printf("%d", myUnion.integerValue);

Arrow Notation:

union MyUnion *ptrToUnion;
ptrToUnion = &myUnion;
ptrToUnion->floatValue = 3.14f;
printf("%f", ptrToUnion->floatValue);

Size and Memory Allocation

The size of a union is determined by the size of its largest member variable. For example, if an integer requires 4 bytes and a float requires 4 bytes, the union will have a size of 4 bytes. This means that the memory allocated for a union is sufficient to hold the largest member only.

It’s important to note that when you change the value of one member in a union, it affects all other members because they share the same memory location.

Use Cases

Unions are commonly used in scenarios where you need to represent data in different formats or save memory.

  • Data Serialization: When serializing data for storage or transmission, unions can be used to represent different types of data in a compact way.
  • Variant Data: Unions can be used to store variant data, where only one member is valid at any given time. This allows you to save memory by reusing the same storage space for different types of data.
  • Bit Manipulation: Unions can be used for bitwise operations, allowing you to manipulate individual bits within a larger data type.


A union is indeed a type of data structure that allows multiple types of data to be stored in the same memory location. It provides flexibility and efficiency when working with diverse or variant data. However, caution must be exercised when accessing union members to ensure proper interpretation of the stored values.

With their ability to save memory and represent multiple types of data, unions are an important tool in programming languages like C and C++. Hopefully, this article has provided you with a better understanding of unions and their use cases.

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